1. Field of the Invention
The invention is concerned with the use of superconducting electronics with a hybrid photonic analog to digital converter to achieve high conversion speeds. The invention differs from most current designs by combining two systems, photonic and superconducting, to utilize each system for optimal performance.
2. Description of the Related Prior Art
Electronic analog-to-digital converter (ADC) has evolved into several different architectures. Today""s popular high-speed flash ADCs interleave parallel channels of sampling and comparing circuits to increase their effective speed by the number of parallel channels (See M. J. Demler, High Speed analog to digital conversion. San Diego: Academic Press, 1991.) The pursuit of even higher speeds along with the limitations of traditional electronic techniques has drawn the superconducting electronics and the photonic communities into this arena. Superconducting electronics combine two important properties for analog-to-digital conversion: quantization and high-speed Josephson digital electronics. Photonic systems lend themselves to ADCs with their large bandwidths and low-noise operation, and can be directly substituted for their electronic counterparts, improving the integrated system and extending the overall performance.
The concept of a photonic ADC spans more than two decades (See H. F. Taylor, xe2x80x9cAn optical analog-to-digital converter-design and analysis,xe2x80x9d IEEE J Quantum Electron., vol. QE-15, pp. 210-16, 1979; see also :R. A. Becker, C. E. Woodward, F. J. Leonberger, and R. C. Williamson, xe2x80x9cWide-band electrooptic guided-wave analog-to-digital converters,xe2x80x9d Proc. IEEE, vol. 72, pp. 802-19, 1984; see also S. Bhushan, F. Coppinger, and B. Jalali, xe2x80x9cTime-stretched analogue-to-digital conversion,xe2x80x9d Electron. Lett., vol. 34, pp. 1081-3, 1998; see also T. R. Clark, J. U. Kang, and R. D. Esman, xe2x80x9cPerformance of a time-and wavelength-interleaved photonic sampler for analog-digital conversion,xe2x80x9d IEEE Photon. Technol. Lett., vol. 11, pp. 1168-70, 1999; and also by this inventor, M. Currie, T. R. Clark, and P. J. Matthews, xe2x80x9cPhotonic analog-to-digital conversion by distributed phase modulation,xe2x80x9d IEEE Photon. Technol Lett., vol. 12, pp. 1689-91, 2000.
Mode-locked lasers employed in photonic ADCs enhance the system by providing picosecond and sub-picosecond sampling of the electronic waveform in an electro-optic device. Photonic sampling functions either as a high-speed, photonic sample-and-hold circuit or as a pre-sampler for an electronic sample-and-hold circuit. A second major attribute of photonic systems also arises from the mode-locked laser source in which the extremely precise timing characteristics of mode-locked lasers serve as an optical clock. Phase noise measurements of such laser systems have exhibited a timing jitter of less than 10 fs for a 10-GHz clock (See T. R. Clark, T. F. Carruthers, P. J. Matthews, and I. N. Duling, III, xe2x80x9cPhase noise measurements of ultrastable 10 GHz harmonically modelocked fibre laser,xe2x80x9d Electron. Lett., vol. 35, pp. 720-1, 1999). 
Superconducting ADCs also enjoy a long history with designs for parallel (flash) and serial (sigma-delta, counting, etc.) ADCs. (See C. A. Hamilton, F. L. Lloyd, and R. L. Kautz, xe2x80x9cSuperconducting A/D converter using latching comparators,xe2x80x9d IEEE Trans. Mag., vol. MAG-21, pp. 197-9, 1985; G. S. Lee and D. A. Peterson, xe2x80x9cSuperconductive A/D converters,xe2x80x9d Proc. IEEE, vol.77, pp.1264-73, 1989; S. V. Rylov, L. A. Bunz, D. V. Gaidarenko, M. A. Fisher, R. P. Robertazzi, and O. A. Mukhanov, xe2x80x9cHigh resolution ADC system,xe2x80x9d IEEE Trans. Appl. Supercond., vol. 7, pp. 2649-52, 1997; S. B. Kaplan, P. D. Bradley, D. K. Brock, D. Gaidarenko, D. Gupta, W. Q. Li, and S. V. Rylov, xe2x80x9cA superconductive flash digitizer with on-chip memory,xe2x80x9d IEEE Trans. Appl. Supercond., vol. 9, pp. 3020-3025, 1999.) 
These converters run the gamut from high-resolution to high-speed. Superconducting ADCs are particularly attractive due to their low power requirements and high-speed operation. Optical coupling to superconducting electronics allows good thermal isolation while also enabling a high-speed interface.
An object of this invention is to provide a hybrid analog to digital converter that uses superconducting electronics with a photonic system to achieve high conversion speeds.
Another object of this invention is for the photonic system to provide optical sampling of the signal with very low aperture and jitter errors.
Another object of this invention is for the optically sampled data to be converted into an electronic signal via an optoelectronic switch.
Another object of the invention is for the converted electronic signal to be quantized by a superconducting system.
Another object of the invention is that the high resolution sampling is provided in the photonic system and not in the superconducting electronics.
Another object of the invention is to provide superconducting quantization of modulated optical pulse trains.
Another object of the invention is to provide an analog to digital converter with a low optical power level of 2 mW at 100 GHz operating speed.
Another object of the invention is that any pulsed laser system can be used and any type of superconducting logic can be used, and any optoelectronic switch can be used to convert the optical pulse train into an electrical pulse train.